Platinum plating composition and process



May 16, 1961 R. DUVA ETAL 2,984,604

PLATINUM PLATING COMPOSITION AND PROCESS Filed Aug. e, 1958 lid 0INVENTORs IFOBERT .P :1 M4 [aw/v (mm-u 1mm? United States PatentPLATINUM PLATING COMPOSITION AND PROCESS Robert Duva, Paramus, and EdwinC. Rinker, Morristown, N.J., assignors to Sel-Rex Corporation, Nutley,N.J., a corporation of New Jersey Filed Aug. 6, 1958, Ser. No. 753,525

5 Claims. (Cl. 204-47) This invention relates to a platinum electrolytefrom which it is possible to plate thick layers of stress free platinumand to the process for making the same.

Up to the present time the only commercially useful platinum platingbaths have depended on chloroplatinic acid (H PtCl -6H O) or platinumdiamino dinitrite Pt(NH (NO for the platinum ions. The chloroplatinicacid baths, though suitable for flash plating, are unwieldy for heavierdeposits, which deposits become spongy as the thickness is increased.The useful life of this bath is very short because of the rapidaccumulation of chloride ions as the platinum metal is replenished.Platinum diamino dinitrite (also known as P-salt) which is disclosed inUS. Patent No. 1,779,436 is dissolved in an electrolyte containingammonium hydroxide. This bath should be maintained at a temperature of200 F. which causes a rapid loss of ammonia and a subsequent loss ofelectrodeposition efliciency. In order to obtain heavier than flashdeposits with these baths it is necessary to remove the article from thebath from time to time, scratch brush it and only then continue theplating. Thcse baths operate erratically, apparently due to inconsistentcathode efficiency and (in the case with the P-salt bath) occasionallycease operating altogether.

Among the objects of this invention is to provide a platinum containingelectrolyte free of the defects of the prior art baths and suitable forplating stress-free platinum of any thickness desired.

Among other objects of the invention is to provide a platinum platingbath suitable for making electroformed platinum articles plated in onecontinuous operation.

Among other objects of the invention is to provide a method for makingthe electrolyte of the invention.

One phase of this invention is based on the discovery that when theP-salt, Pt(NH (NO) is heated with an aqueous solution of sulfamic acidit dissolves therein.

Another phase of the invention is based on the discovery that thesolution obtained by heating the P-salt with the sulfamic acid can bediluted to provide a stable electrolytic solution for electroplatingstress free platinum or for electroforming platinum articles.

Thus, the objects of the invention are attained by heating the platinumdiamino dinitrite in a sulfamic acid aqueous solution in the proportionsof about 10-40 or more g. of P-salt per -200 cc. of sulfamic acidsolution containing about 2 to about 400 g./liter of sulfamic acid,until a clear solution is obtained. Thereafter, the solution is dilutedto provide a solution containing at least about 6 g. per liter ofplatinum (which corresponds to about 10 g. per liter of the P-salt).Articles are then electroplated or electroformed from this solution byconventional processes.

Plating of thick deposits of platinum in this low stress bath can bedone in one operation, i.e., there is no need for intermittently scratchbrushing or burnishing to produce a smooth deposit. Platinum platingwith this low stress bath can be performed on a specification basis,i.e.,

constant electrodeposition eificiency is obtained at constanttemperature and platinum metal content. For example, uniform depositswill be obtained on a series of articles by electroplating each in thebath for one minute.

The following are very satisfactory operating conditions for such abath.

Platinum metal content- 6-20 grams/ liter or more. Sulfamic acid 20-100grams per liter.

Bath container Pyrex glass.

Anodes Platinum.

Anode to cathode ratio..- 1 to 1 or higher.

Operating temperature-.. 65 to 100 C.

Current density 20-100 amperes per sq. foot. Agitation rate Moderate.Electrodeposition rate 5-30 milligrams per ampere minute.

The sulfamic acid above is calculated as pure acid although it may beadded as a solution, if desired.

Platinum metal replenishment is accomplished by the addition of theelectrolyte as such to maintain the platinum metal at the desiredconcentration. Solution level is maintained by the additiot f both thisreplenisher and water.

Below 6 -g./l. of platinum metal the deposits are .spongy. There is,however, no actual upper limit for the platinum metal content exceptpossibly the limit of solubility of the P-salt. No advantage is obtainedby increasing the amount of Pt above 20 g./l., however, since even at 15g./l. (at 99 C.) an efficiency of about 30 milligrams per ampere minuteis attained which is practically the theoretical maximum of about 30.4mg/amp. On the other hand, increasing the Pt content also increases thelosses due to drag out.

The drawing is a reproduction of a photomicrograph of a specimen ofplatinum plated graphite made according to the invention.

The following example illustrates how the invention 15 put intopractice.

Example 50 grams of a sulfamic acid (NH SO H) in 200 cc. of water isheated to about. C. whereupon 20 g. of platinum diamino dinitrite areadded with stirring. The stirring and heating is continued until a clearsolution is obtained. It has not been possible to determine thecomposition of the resulting salt in the solution at the present timebut it is probably a fairly complex compound. The resultant solution isdiluted with water to make 1 liter of solution and is then ready forelectroplating.

The platinum is plated at a voltage of about 1 volt to 3 volts and at acurrent density of about 20 amps. per square foot.

The proportion of sulfamic acid of P-salt can vary from about 1:10 toabout 20:1. The electroplating bath should have a concentration of atleast about 6 g. of Pt per liter for otherwise spongy deposits result.

The bath of the invention is stable as long as it is operated.

In electroforming conventional practice is followed. The form may becoated with resist material where no plating is desired, the plating onthe exposed surface is continued without interruption until the desiredthickness is built-up and thereafter the original form is separated fromthe plated deposit. With this bath, the removal of the basis metal fromthe deposit results in the production of a continuous film of platinummetal showing the lack of internal stress of the plated metal. In otherwords the platinum metal does not disintegrate into flakes or smallpieces. With the process of the invention as described in Example 1, anadherent layer of platinum has been plated on graphite. The throwingpower of the bath is excellent as can be seen from the fact that theplatinum penetrated into the pores of the graphite as shown in thedrawing. A protective layer of nickel was deposited thereon beforecutting the section which was photographed.

The features and principles underlying the invention described above inconnection with specific exemplifications will suggest to those skilledin the art many other modifications thereof. It is accordingly desiredthat the appended claims shall not be limited to any specific feature ordetails thereof.

We claim:

1. A process for electroplating. relatively thick layers of stress-freeplatinum comprisingdissolving platinum diamino dinitrite inan aqueoussolution of sulfamicacid containing a sufiicient amountof sulfamic acidto dissolve said platinum diamino dinitrite, adding waterto theresultant solution to provide a solution containingat least 6 g./ literof platinum metal andelectrolyzing said solution to plate out platinumtherefrom.

.2. The process as claimed in claim 1 comprising forming the aqueoussolution by heating the acid solution to dissolve the platinum salt.

3. As an electrolyte for the plating of platinum the compositioncoinprising water and the solution obtained by heating about 10-40 g. ofplatinum diamino dinitrite with 15-200 cc. of aqueous sulfamic acidsolution containing about 2 to about 400 grams per liter of sulfamicacid.

4. A process for electroplating relatively thick layers of stress freeplatinum comprising providing a solution of about 10-40 g./liter ofplatinum diamino dinitrit'e in 15200 cc. of aqueous sulfamic acidsolution containing about 2 to about 400 g./liter of sulfamic acid,adding water to said solution to provide a solution containing 6 g./liter of platinum and electrolyzing said solution to plate out platinumtherefrom. I V

5. The electrolyte as claimed in claim 3, wherein said electrolytecomprises at least about 6 g./liter of platinum metal.

References Cited in the file of this patent UNITED STATES PATENTS1,779,436 Keitel Oct. '28, 1930 1,921,941 Powell et a1. u .7...... Aug.8, 1933 2,027,358 Powell et al Jan. 7, 1936

1. A PROCESS FOR ELECTROPLATING RELATIVELY THICK LAYERS OF STRESS-FREEPLATINUM COMPRISING DISSOLVING PLATINUM DIAMINO DINITRITE IN AN AQUEOUSSOLUTION OF SULFAMIC ACID CONTAINING A SUFFICIENT AMOUNT OF SULFAMICACID TO DISSOLVE SAID PLATINUM DIAMINO DINITRITE, ADDING WATER TO THERESULTANT SOLUTION TO PROVIDE A SOLUTION CONTAINING AT LEAST 6 G./LITEROF PLATINUM METAL AND ELECTROLYZING SAID SOLUTION TO PLATE OUT PLATINUMTHEREFROM.